The invention relates to a laser optical system and to a diode laser.
As opposed to conventional laser beam sources, which have a beam diameter of a few mm with a low beam divergence in the range of a few mrad, the beam of a semiconductor laser or diode laser (hereinafter “diode laser”) is characterized by a highly divergent beam in the fast axis with a divergence >1000 mrad. This is caused by the exit layer which is limited to <1 μm in height, on which, similar to diffraction on a gap-like opening, a greater divergence angle is produced. Since the extension of the exit opening in the plane is different perpendicular and parallel to the active semiconductor layer, various beam divergences occur in the plane perpendicular and parallel to the active layer.
To achieve a power output of 20-40 W for a diode laser, numerous laser emitters are combined on what is known as a laser bar to form a laser component. Commonly, 10-50 individual emitter groups are arranged in a row in the plane parallel to the active layer. The resulting beam of such a bar in the plane parallel to the active layer has an opening angle of ca. 10° and a beam diameter of ca. 10 mm. The resulting beam quality in this plane is several times less than the resulting beam quality in the above described plane perpendicular to the active layer. Also in the event of a possible future reduction of the divergence angle of laser chips the ratio of the greatly differing beam quality perpendicular and parallel to the active layer remains.
The beam exhibits a significant difference of beam quality in the two directions perpendicular and parallel to the active layer based on the beam characteristic described above. The beam quality is defined by the M2 parameter. M2 is defined by the factor with which the beam divergence of the diode laser beam exceeds the beam divergence of a diffraction-limited beam with the same diameter. In the case described above, in the plane parallel to the active layer there is a beam diameter which exceeds the beam diameter in the vertical plane by a factor of 10,000. With respect to the beam divergence the behavior is somewhat different, i.e. in the plane parallel to the active layer or in the slow axis the beam divergence achieved is less by almost a factor of 10. The M2 parameter in the plane parallel to the active layer is thus several orders of magnitude above the M2 value in the plane perpendicular to the active layer.
A possible goal of beam shaping is to achieve a beam with nearly the same M2 values in both planes, i.e. perpendicular and parallel to the active later. The following methods are currently known for shaping the beam geometry, by which an approximation of the beam qualities in the two main planes of the beam is achieved.
By means of a fiber bundle, linear beam cross sections can be combined to form a circular bundle by rearranging the fibers. Such processes are described for example in U.S. Pat. Nos. 5,127,068, 4,763,975, 4,818,062, 5,268,978 and 5,258,989.
In addition, there is the technique of beam rotation, in which the radiation of single emitters is turned by 90° in order to achieve a rearrangement, in which an arrangement of the beams in the direction of the axis of better beam quality takes place. The following arrangements are known for this process: U.S. Pat. No. 5,168,401, EP 0 484 276, DE 4 438 368. A common characteristic of all processes is that the radiation of a diode laser, after its collimation in the fast axis direction, is turned 90° in order to achieve slow axis collimation with a common cylinder optical component. In a modification of this process a continuous line source is also conceivable (for example, that of a diode laser of high surface density which is collimated in the fast axis direction) with a beam profile (line) which is divided and which is present in rearranged form behind the optical element.
In addition, it is possible to achieve rearrangement of the radiation of individual emitters without rotating the beam, rearrangement of the radiation being achieved for example by parallel offset (shifting) by means of parallel mirrors (WO 95/15510). An arrangement which likewise uses the technique of rearrangement is described in DE 195 00 53 and DE 19 5 44 488. In this case, the radiation of a diode laser bar is deflected into different planes and is individually collimated there.
The disadvantages of the existing art can be summarized in that for fiber-coupled diode lasers, usually a beam with very different beam qualities in two axial directions is coupled into the fibers. In the case of a round fiber, this means that the possible numeric aperture or the fiber diameter is not used in one axial direction. This causes significant losses in power density, so that in practical application there is a limitation to ca. 104 W/cm2.
Furthermore, in the known processes described above, to some extent considerable differences in the path length must be compensated. This is generally achieved by correction prisms, which can compensate for errors only to a limited extent. Furthermore, multiple reflections necessitate increased requirements for adjustment precision, production tolerances and component stability (WO 95/15510). Reflecting optical components (made of copper, for example) have high absorption values. Also known in the art is a laser optical system of the generic type for shaping at least one laser beam bundle, using at least two optical shaping elements provided consecutively in the laser beam, of which at least one shaping element is embodied as a plate fan (DE 197 05 574 A1).
It is an object of the invention is to provide a laser optical system that enables improved focusing of the laser beam.